Your story reminded me of something my sister did last week. She is two years younger then me and visiting eastern Canada/me on vacation. I met her down in Toronto and we went out clubbing with a bunch of her friends. She then proceeded to give MY phone number out to random guys. Goddamn sister! Its a good thing I love her.
Anyhow, I think the idea is how much do you trust your sister? If she is a decent girl she should be able to make good choices about who to sleep with. Just accept it.
As for sisters and friends I feel that sisters are off limits as it is just a disaster waiting to happen. Friendship is far more important then any woman.